As an oil supply apparatus of an internal combustion engine to be mounted on a vehicle for supplying oil for lubricating and cooling the oil lubrication sections to oil lubrication sections of the internal combustion engine, there has so far been used an oil pump which has a variable discharge amount structure capable of suitably adjusting oil discharge pressure in response to the rotation speed of the internal combustion engine (see for example Patent Document 1).
The oil pump of this kind is generally provided with a pump body which comprises a suction port, a main discharge port, and a sub discharge port. The suction port allows the oil stored in an oil pan to be suctioned therethrough in response to the rotation of a rotor to be driven in synchronism with a crankshaft forming part of the engine, while the main discharge port and the sub discharge port permits the oil to be discharged therethrough in response to the rotor. The oil pump has a first oil passage for allowing the oil from the main discharge port to be supplied to the lubrication sections, a second oil passage for allowing the oil from the sub discharge port to be supplied to the first oil passage, and a relief passage for returning to at least one of the suction port and the oil pan the oil from a hydraulic pressure control valve provided with a valve movable in response to the pressure of the oil to the first oil passage.
The valve of the oil pump is formed with a first valve oil passage and a second valve oil passage. The valve of the oil pump is constructed to supply the oil from the sub discharge port to the first oil passage by way of the first valve oil passage when the pressure of the oil to the first oil passage is in the range of a predetermined pressure level, and on the other hand to supply the oil from the sub discharge port to the first oil passage by way of the second valve oil passage when the pressure of the oil to the first oil passage exceeds predetermined pressure level.
The oil pump constructed to enable the oil from the sub discharge port to be supplied to the first oil passage by way of the first valve oil passage when the pressure of the oil to the first oil passage is in the range of a predetermined pressure level leads to the fact that the supply amount of oil to the first oil passage is equal to an amount totaling the discharge amounts of the oil through the main discharge port and the sub discharge port, respectively, as shown by the solid line O-P in FIG. 25
If the internal combustion engine is operated to increase its rotation speed and thereby to secure a sufficient amount of oil to be supplied to the lubrication sections only with the oil passing through the main discharge port, the oil pump is operated to have a surplus amount of oil in the second oil passage not supplied to the first oil passage but returned to the relief oil passage (see the lines P-Q and Q-R in FIG. 25), resulting from the fact that the oil from the first oil passage is not required to be merged with the oil from the second oil passage.
On the other hand, the internal combustion engine operated at a high rotation speed has lubrication sections required to be supplied with a large amount of oil, i.e., a sufficient amount of oil. For this reason, the oil pump is constructed to supply the oil from the sub discharge port to the first oil passage by way of the second valve oil passage (see the line R-S in FIG. 25) when the pressure of the oil to the first oil passage exceeds predetermined pressure level.
At this time, the oil pump can have the amount of oil to be supplied to the first oil passage again totaling the discharge amounts of the oil through the main discharge port and the sub discharge port, even after the amount of oil to be supplied to the first oil passage is once only the amount of oil passing through the main discharge port.
The oil pump has an oil pressure property characterized by the oil pressure vertically increased from a changed discharge pressure R to a changed discharge pressure S when the rotation speed of the internal combustion engine is increased to a target rotation speed N3 as shown by the solid line R-S in FIG. 25.
As a consequence, the oil pump can secure a sufficient oil discharge pressure, i.e., a sufficient amount of oil required to be supplied to the lubrication sections due to the fact that the volume of oil can drastically be increased even when the internal combustion engine is operated at a high rotation speed.
The conventional oil pump has an increase rate of the oil discharge pressure of the oil pump per the rotation speed of the internal combustion engine which is varied to have a plurality of different stages (O-P, P-Q, Q-R, R-S, S-T, T-U) of discharge pressure in the rotation speed area (O-N1, N1-N2, N2-N3, N3-N4, N4-N5) as shown in FIG. 25. The conventional oil pump is controlled to have the oil discharge pressure variable in such a manner that the increase rate of the oil discharge pressure can be varied to have the plurality of different stages of discharge pressure in the rotation speed area, thereby making it possible to supply an optimum amount of oil to the lubrication sections in response to the rotation speed of the internal combustion engine.
The oil pump is set to have an increase rate of the oil discharge pressure of the oil pump variable at the plurality of different stages of discharge pressure in the rotation speed area, and enables the oil discharge pressure to be decreased to the line P-Q-R in the intermediate rotation area (N1-N3) of the internal combustion engine. This means that the oil pump can supply to the lubrication sections the amount of oil more than needed, thereby making it possible to prevent the oil pump from being applied with superfluous load increased thereto.
On the other hand, the oil stored in the oil pan is supplied to the lubrication sections by the oil pump to lubricate and cool the lubrication sections, and thereafter is collected to the oil pan. At this time, the amount of oil to be stored in the oil pan being decreased leads to the deterioration of the collection ratio of the oil to be collected in the oil pan, thereby giving rise to lowering the oil level of the oil from the optimum amount of oil to be stored in the oil pan, viz., lacking the oil in the oil pan.
The oil level lowered as previously mentioned results in the fact that there is a possibility that a strainer is brought into a state in which the strainer sucks air when allowing the oil to pass therethrough from the oil pan to the oil pump (this state being hereinafter simply referred to as “air sucking state”).
The strainer held in such an air sucking state results in the reduction of the oil discharge pressure to be discharged from the oil pump, thereby giving rise to a possibility that the pump cannot supply a sufficient amount of oil to the lubrication sections, and thereby leading to deteriorating a lubrication property to the lubrication sections.
The conventional oil pump is required to promptly increase the changed discharge pressure S for example when the oil discharge pressure is changed to the changed discharge pressure R. When the air is sucked from the strainer, the changed discharge pressure R is shifted to the high rotation speed side from the rotation speed N3 of the internal combustion engine. Therefore, the rotation speed for the purpose of raising the oil discharge pressure to the changed discharge pressure R is shifted to the high rotation speed side, so that the oil pump cannot supply a sufficient amount of oil to the lubrication sections to which a large amount of oil is required to supply the oil when the internal combustion engine is operated in a high rotation speed area.
To solve the foregoing drawbacks, the conventional oil supply apparatus is constructed to detect the oil level of the oil stored in the oil pan and to monitor whether or not the oil is lowered from the desired oil level, thereby enabling the oil to be replenished if necessary.
One of the known oil supply apparatuses is provided with an oil level sensor to detect the oil level of the oil stored in the oil pan (see for example Patent Document 2). This sensor comprises a floating object, and a link mechanism, so that the sensor can detect the floating object downwardly moved together with the oil level, thereby making it possible to detect the lowering of the oil level.
An additional oil supply apparatus is known as being provided with an oil level gauge for detecting the oil level in the oil pan (see for example Patent Document 3).
The oil level gauge is attached to the internal combustion engine in a state in which the oil level gauge is inserted into a through-bore formed in the internal combustion engine, and has a leading end portion soaked in the oil stored in the oil pan.
The above known oil supply apparatus of the internal combustion engine can confirm the oil level and the state of the oil in the oil pan by watching the oil adhered to the leading end portion of the oil level gauge after the oil level gauge is removed from the internal combustion engine.